1. Field of Invention
The present invention relates generally to acoustic spectrometry and specifically to the use of acoustic spectrometry to verify the geometry of a cavity within a cast, molded or machined part.
2. Description of Prior Art
A cavity within a cast, molded or machined part often needs to be inspected for defects before additional manufacturing steps add value to a part that may need to be scrapped or reworked. The function of the part may critically depend on the shape and size of the cavity. Positively and quickly identifying defective parts is important in a manufacturing environment. Building assemblies with defective parts may result in shortened product lifetime or poor product performance. Significant prior art has attempted to address this need, however all fall short of solving the problem.
U.S. Pat. No. 1,884,042, Indicator, teaches the use of forcing high-pressure gas through a cavity to be tested and measuring backpressure. Backpressure different from an expected value would result in rejection. While this technique may find major blockages, it is limited to finding blockages that constrict the cavity to a passage smaller than the smallest expected constriction in the cavity and it cannot determine anything at all about blind passages within the cavity. Internal cavities may consist of a complex of interconnecting passages and many defects cannot be detected by this method.
U.S. Pat. No. 2,666,326, Volumetric Measuring Apparatus, teaches a way of detecting volume defects in a cavity by creating a Helmoltz resonator, which measures the volume of the cavity. This technique is good for detecting volume related defects, but will not detect a blockage in a more complicated cavity. Many kinds of shape defects cannot be detected with this apparatus.
U.S. Pat. No. 5,109,699, Sonic Casting Tester, teaches an apparatus and method of blockage detection whereby acoustic wave amplitude attenuation is used to determine a blockage. This technique is again limited to blockages that constrict a passage to a smaller cross section than the smallest cross section expected in the cavity. Shape defects other than blockages of the cavity are not readily detected by this technique.
U.S. Pat. No. 4,480,473, Acoustic Inspection Method, teaches a way of acoustically testing an internal fluid flow path for proper path length, by measuring the phase difference of an acoustic signal traversing the fluid flow path and comparing that to a known good path. Partial blockages may not be detected with this technique, in addition it is not well suited to testing complex cavities that contain blind passages or have multiple paths from input to output.
What is needed is a cavity tester that can test cavity geometry in a more general way and requires little setup.
It is therefore an object of the present invention to provide an apparatus and method for reliably detecting blockages and other geometric anomalies in a cavity within a cast, molded or machined part.
It is a further object of the present invention to provide such an apparatus and method using acoustic techniques.
It is a further object of the present invention to provide such an apparatus and method requiring little or no qualitative decisions that may be completely automated.
It is a further object of the present invention to provide such an apparatus and method that detects defects within blind passages.
It is a further object of the present invention to provide such an apparatus and method that detects missing or incorrectly drilled access holes in such internal cavities.
It is a further object of the present invention to provide such an apparatus and method that does not require a fixture that is inserted into the cavity under test.
It is a further object of the present invention to provide such an apparatus and method that can test multiple unconnected cavities together all at once.
Still other objects and advantages of the present invention will become apparent to those of ordinary skill in the art having reference to the following specification together with its drawings.
To achieve the foregoing and other objects, according to the main aspect of the present invention, there is provided a method and apparatus for testing the geometry of cavities within a cast, molded or machined part.
In the preferred embodiment of the present invention, the cavity under test is sealed and an acoustic wave is introduced into one end of the cavity. At a second end of the cavity, the amplitude and phase shift of the acoustic wave are measured at a multiplicity of predetermined frequencies. The resulting acoustic spectrum is compared to the spectrum obtained from a known good part.
In another embodiment of the present invention, access holes cast or drilled through the part into the cavity are covered with closed-end tubes. Measuring the acoustic spectrum as in the preferred embodiment now allows the access hole sizes to be compared to those of a reference part along with the geometry of the rest of the cavity.
In another embodiment of the present invention, multiple unconnected cavities in a part are combined into one cavity with a manifold such that the geometry of all cavities may be tested at once.